1. Field of the Invention
The present invention relates to a liquid crystal display, and more particularly, to a thin film transistor liquid crystal device and a method of fabricating the same. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for easily detecting a short line defect, thereby increasing yield in fabricating a liquid crystal display.
2. Discussion of the Related Art
A liquid crystal display (LCD) has been widely used in office automation equipment and video units because it has advantages in size and power consumption. Such an LCD typically utilizes an optical anisotropy of a liquid crystal (LC). The LC has thin and long shape molecules, which causes an orientation alignment of the LC molecules. Therefore, an alignment direction of the LC molecules are be controlled by applying electric fields to the LC molecules. When the alignment direction of the LC molecules are properly adjusted, the LC is aligned and light is reflected along the alignment direction of the LC molecules in displaying image data.
An active matrix (AM) LCD having a plurality of thin film transistors (TFTs) and pixel electrodes arranged in the shape of an array matrix draws most attention because of its high resolution and superiority in displaying moving pictures. The active matrix type liquid crystal display device, employing a TFT as a switching device, is formed on an array substrate having a matrix array of TFTs and pixel electrodes, and an opposing substrate arranged opposing the TFT substrate, with a liquid crystal material interposed therein. The opposing substrate includes a light-shielding film (so called a black matrix), a color filter and a common electrodes.
Now, referring to the attached drawings, a conventional back-channel-etching type structure of an array substrate of the liquid crystal display device manufactured by a conventional method is explained in detail. As shown in FIG. 1, a liquid crystal display 20 includes an array substrate 2, a color filter substrate 4 facing into the array substrate 2, a liquid crystal 10 interposed between the array and color filter substrates 2 and 4, and a sealant 6 formed at the periphery of the gap between the two substrates 2 and 4. The sealant 6 prevents the liquid crystal 10 from leaking out of the liquid crystal display device 20.
The array substrate 2 includes a substrate 1, a thin film transistor xe2x80x9cSxe2x80x9d as a switching element for changing an orientation of the liquid crystal 10, and a pixel electrode 14 as a first electrode for applying electric fields to the liquid crystal 10. The color filter substrate 4 includes another substrate 1, a color filter 8 for displaying colors, and a common electrode 12 as a second electrode for applying electric fields to the liquid crystal 10. A pixel region xe2x80x9cPxe2x80x9d includes the pixel electrode 14 and serves as a display area for displaying images.
Referring to FIG. 2, a detailed description of the structure and operation of the array substrate 2 will be followed. On the substrate 1, a gate line 22 is transversely formed with a data line 24 arranged perpendicular thereto, and a pixel electrode 14 is formed on the area defined by the gate and data lines 22 and 24. Near the cross point between the gate and data lines 22 and 24, a portion of the gate line 22 is used as a gate electrode 26. Further, near the cross point between the gate and data lines 22 and 24, a source electrode 28 is protruded from the data line 24 spaced apart from the source electrode 28 and a drain electrode 30 is formed. The TFT xe2x80x9cSxe2x80x9d includes the gate electrode 26, the source electrode 28, and the drain electrode 30 as well as an active layer 55, which usually serves as the most important element to switch the liquid crystal layer (reference numeral 10 of FIG. 1). Since it is easy to form an amorphous silicon layer at a relatively low temperature such as below 350xc2x0 C., amorphous silicon is conventionally used for the active layer 55.
While not shown in FIG. 2, data pads and gate pads are respectively formed at each end of the data lines and gate lines. The data and gate pads (not shown) electrically connect the TFT xe2x80x9cSxe2x80x9d and the pixel electrode 14 with corresponding external driving circuits (not shown), respectively. The TFT xe2x80x9cSxe2x80x9d serves as a switching device to apply signals for the pixel electrode 14.
Still referring to FIG. 2, a drain contact hole 34 is formed over the drain electrode 30. The pixel electrode 14 (shown in FIG. 1) electrically contacts the drain electrode 30 through the drain contact hole 34. A capacitor electrode 21 is integrally formed with the gate line 22, and overlaps the pixel electrode 14 to form a storage capacitor Cst. The storage capacitor Cst serves to store electric charges.
As explained previously, the TFT xe2x80x9cSxe2x80x9d including the gate, source, and drain electrodes 26, 28 and 30 acts as a switch for applying electric fields to the liquid crystal 10 (shown in FIG. 1). That is a say, in operation, if only signals are applied to the gate electrode 26 of the TFT xe2x80x9cSxe2x80x9d, the electric fields are applied to the pixel electrode 14. The signals applied to the TFT xe2x80x9cSxe2x80x9d are transmitted from an outer circuit (not shown), which is electrically connected with the data and gate pads (not shown).
A fabricating process of the above-mentioned array substrate requires repeated steps of depositing and patterning of various layers. The patterning step adopts a photolithography mask step including light exposing with a mask. Since one cycle of the photolithography step is facilitated with one mask, the number of masks used in the fabrication process is a critical factor in determining the number of patterning steps. As the fabricating process for the array substrate becomes simpler, errors may decrease. The fabricating process for the array substrate is set out according to design specifications for the array substrate or materials used for the various layers in the array substrate. For example, in case of fabricating a large-scaled (12 inches or larger) LCD, a specific resistance of a material for the gate lines serves as a critical factor in determining the quality of the LCD. Therefore, a highly conductive metal such as aluminum (Al) or aluminum alloys are usually used for the large scaled LCD device.
Referring now to FIGS. 3A to 3E, a fabrication method and a more detailed description of the structure of the TFT and the storage capacitor will be discussed as follows. For the TFT, an inverted staggered type is widely employed due to its advantages of a simple structure and a superior quality. The inverted staggered tube TFT is divided into a back-channel-etch type and an etching-stopper type according to a method of forming a channel in the TFT. The back-channel-etch type has a simpler structure. FIGS. 3A to 3E refer to the back-channel-etch type TFT.
At first, a glass substrate 1 is cleaned to remove particles or contaminants on the surface of the substrate 1. Then, as shown in FIG. 3A, a first metal layer is deposited on the substrate 1 and patterned by lithography to form the gate electrode 26, the gate line (reference numeral 22 of FIG. 2), and the capacitor electrode 21. Aluminum is widely used for the gate electrode 26 to decrease a RC delay. However, a pure aluminum is chemically weak and may involve an occurrence of a hillock in a high-temperature process. Therefore, aluminum alloys or a layered aluminum is used for the gate electrode instead of a pure aluminum.
Next, as shown in FIG. 3B, a gate insulating layer 5C is formed on the substrate 1 to cover the first patterned metal layer including the gate electrode 26 and the capacitor electrode 21. Thereafter, an amorphous silicon layer 52 (a-Si:H) and a doped amorphous silicon layer 54 (n+a-Si:H) are sequentially formed on the gate insulating layer 50. The silicon layers 52 and 54 are patterned to form the active layer 55. The doped amorphous silicon 54 serves as an ohmic contact layer to decrease a contact resistance measured between the active layer 55 and a metal layer that will be formed in a later step.
Next, as shown in FIG. 3C, a metal layer is deposited and patterned to form the source and drain electrodes 28 and 30. At this point, the data line 24 is integrally formed with the source electrode 28. Thereafter, using the source and drain electrodes 28 and 30 as masks, a portion of the doped amorphous layer 54, an ohmic contact layer, is etched to form a channel xe2x80x9cCHxe2x80x9d between the source and drain electrodes 28 and 30. Unless the ohmic contact layer between the source and drain electrodes 28 and 30 is etched, a critical problem may occur in an electrical characteristic of the TFT (reference xe2x80x9cSxe2x80x9d of FIG. 2).
However, since there is no etching selectively between the ohmic contact layer 54 and the amorphous silicon layer 52, it should be very careful in etching the ohmic contact layer 54 between the source and drain electrodes 28 and 30. Actually, about 50 nm of the amorphous silicon layer 52 is further etched in forming the channel xe2x80x9cCHxe2x80x9d between the source and drain electrodes 28 and 30. The characteristic of the TFT xe2x80x9cSxe2x80x9d directly depends on an etching uniformity of the over-etched portion in the amorphous silicon layer 52.
Next, as shown in FIG. 30, an insulating layer is deposited and patterned to form a passivation layer 56, which serves to protect the active layer 55. The passivation layer 56 includes an inorganic material such as silicon oxide (SiO2) or an organic material such as benzocyclobutene (BCB). Those materials have high light-transmittance, water-resistance, and good reliability, which are requirements for the passivation layer 56. In addition, a drain contact hole 34 is formed in the passivation layer 56 to expose a portion of the drain contact hole 30.
Next, as shown in FIG. 3E, a transparent conductive material is deposited and patterned to form the pixel electrode 14. Indium tin oxide (ITO) is usually used for the pixel electrode 14. The pixel electrode 14 electrically contacts the drain electrode 30 through the drain contact hole 34.
FIG. 4 shows the above-described fabricating process as a flow chart. In step 200, the substrate is cleaned in order to remove particles or contaminants on the surface of the substrate.
In step 210, depositing a first metal layer and patterning the first material layer by photolithography form the gate lines including the gate electrodes.
In step 220, a gate insulating layer, a semiconductor layer, and an ohmic contact layer are formed by depositing a first insulating material, the semiconductor material and the doped semiconductor material sequentially and patterning the semiconductor material and the doped semiconductor material. The gate insulating layer conventionally has a thickness of about 3000 xc3x85.
In step 230, depositing and patterning a second metallic layer of chromium (Cr) or a chromium alloy form the source and drain electrodes.
In step 240, a back channel is formed by etching the ohmic contact layer using the source and drain electrodes as masks.
In step 250, depositing and patterning a second insulating layer form a passivation layer and contact holes.
In step 260, depositing and patterning a transparent conductive material form pixel electrodes.
Recently, a resolution and an aperture ratio are more important specification parameters for an LCD. However, as the resolution and the aperture ratio become higher, more errors may occur during the fabricating process for the conventional LCD. FIG. 5 illustrates an example of such errors.
FIG. 5 is a cross-sectional view taken along the line Vxe2x80x94Vxe2x80x2 in FIG. 2. As shown, the data line 24 is formed on the gate insulating layer 50, which is formed on the substrate 1. After a passivation layer 56 is formed to cover the data line 24, a metal layer used for the pixel electrodes 14 and 16 is deposited on the passivation layer 56. A metal layer is patterned to form the pixel electrodes 14 and 16 with the data line 24 centered under the boundary area between the pixel electrodes 14 and 16. At this point, to achieve a high resolution and a high aperture ratio, gaps between the pixel electrodes 14 and 16 must be designed to be very minute. Since the above-mentioned gaps are too minute to be properly patterned, a defected pattern 13 may be left between the pixel electrodes 14 and 16 during the patterning process thereof. The defect pattern 13 results in a short circuit between the different pixel electrodes 14 and 16, which causes a point defect of the LCD.
The above-mentioned point defect due to the short circuit between the pixel electrodes is difficult to detect during a patterning examination because the pixel electrode 14 and 16 are very thin and transparent. For example, an ITO of about a 500 xc3x85 thickness is used for the pixel electrode. Further, no electrical method for precisely detecting the point defect has been developed by now.
FIG. 6 is a graph illustrating a capacitance characteristic of the conventional pixel electrode when an electric signal is applied to the gate and source electrodes of the TFT. When a gate signal xe2x80x9cVgxe2x80x9d and a display signal xe2x80x9cVdxe2x80x9d are respectively applied to the gate and source electrodes at the same time, the display signal xe2x80x9cVdxe2x80x9d transmits through the drain electrode and the pixel electrode and is stored therein until another gate signal is applied to the gate electrode. Therefore, a pixel signal xe2x80x9cVpxe2x80x9d measured from the pixel electrode should maintain its value until another gate signal is applied.
As shown in FIG. 6, a first broken line 100 is a normal pixel signal measured from a normal pixel electrode, whereas a second broken line 102 is an abnormal pixel signal measured from a defective pixel electrode. As shown, with the gate signal xe2x80x9cVgxe2x80x9d applied to the gate electrode, the pixel signal xe2x80x9cVpxe2x80x9d begins to increase and when the display signal xe2x80x9cVdxe2x80x9d is stopped, the pixel signal xe2x80x9cVpxe2x80x9d reaches its peak value, which is the same value as the display signal xe2x80x9cVdxe2x80x9d. In case of the normal pixel electrode, the pixel signal xe2x80x9cVpxe2x80x9d maintains its value as the first broken line 100 shows. However, when a short circuit is generated between the pixel electrodes, the pixel signal xe2x80x9cVpxe2x80x9d leaks to the adjacent pixel electrode. Therefore, the pixel signal xe2x80x9cVpxe2x80x9d decreases by a voltage difference (xcex94V) as the second broken line 102 shows.
Although there is the voltage difference (xcex94V) between the normal pixel signal and the abnormal pixel signal, the voltage difference (xcex94V) is too small to be detected by an electric detector. Accordingly, in spite of the above-mentioned short circuit defect, the defective array substrate is used for fabricating the LCD device, which causes a low yield in fabricating an LCD.
Accordingly, the present invention is directed to a thin film transistor liquid crystal display and method of fabricating the same that substantially obviates one or more of problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide a liquid crystal display having a structure where a short circuit between pixel electrodes is easily detected.
Additional features and advantages of the invention will be set forth in the description, which follows and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a liquid crystal display includes the first and second substrates, a gate line, a data line, and a thin film transistor on the first substrate, a passivation layer on the thin film transistor and the gate line, wherein a data line hole is formed in the passivation layer, thereby exposing a portion the data line, a pixel electrode on the passivation layer, a common electrode on the second substrate, and a liquid crystal layer between the first and second substrates.
In another aspect of the present invention, a method of fabricating a liquid crystal display having first and second substrates, the method comprising the steps of forming a gate electrode on the first substrate, forming a gate line on the gate electrode including the first substrate, forming a gate insulating layer on the gate line including the first substrate, forming an active layer on the gate insulating layer, forming a data line, a source electrode, and a drain electrode on the gate insulating layer, wherein the source and drain electrodes overlap the active layer, forming a passivation layer to cover the source electrode, the drain electrode, the active layer, and the data line, forming a data line contact hole in the passivation layer, wherein the data line hole exposes a portion of the data line, and forming a pixel electrode on the passivation layer.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory are intended to provide further explanation of the invention as claimed.